The present invention relates to integrated circuits and semiconductor device fabrication and, more specifically, to structures that include a spin-torque-transfer magnetic tunnel junction stack and methods for fabricating such structures.
Magnetic random access memory (MRAM) is a non-volatile computer memory technology based on magnetoresistance. Because MRAM is non-volatile, MRAM can maintain memory content when the memory device is not powered. In an MRAM, data is stored by magnetoresistive elements made from a pinned magnetic layer and a free magnetic layer, each of which holds a magnetization. The magnetization of the pinned layer is fixed in its magnetic orientation, and the magnetization of the free layer can be changed by an external magnetic field generated by a programming current. In particular, the external magnetic field can cause the magnetic orientations of the magnetic layers to either be parallel, giving a lower electrical resistance across the layers (“0” state), or antiparallel, giving a higher electrical resistance across the layers (“1” state). The switching of the magnetic orientation of the free layer and the resulting high or low resistance states across the magnetic layers provide for the write and read operations of the MRAM cell.
A spin-torque-transfer MRAM (STT-MRAM) cell may include a magnetoresistive data-storing element in the form of a magnetic tunnel junction (MTJ) that includes a pinned magnetic layer, a free magnetic layer, and an insulating layer between the magnetic layers. The STT-MRAM cell further includes an access transistor, and is coupled with a bit line, a word line, and a source line. A programming current typically flows through the access transistor and the magnetic cell stack. The pinned magnetic layer polarizes the electron spin of the programming current, and torque is created as the spin-polarized current passes through the MTJ. The spin-polarized electron current interacts with the free magnetic layer by exerting a torque on the free magnetic layer. When the torque of the spin-polarized electron current passing through the MTJ is greater than a critical switching current density, the torque exerted by the spin-polarized electron current is sufficient to switch the magnetization of the free magnetic layer. Thus, the magnetization of the free magnetic layer can be aligned to be either parallel or antiparallel to the pinned layer, and the resistance state across the MTJ is changed.
Improved structures including a spin-torque-transfer magnetic tunnel junction stack and methods for fabricating such structures are needed.